Motivated by the need to predict how the Arctic atmosphere will
change in a warming world, this article summarizes recent advances made by
the research consortium NETCARE (Network on Climate and Aerosols: Addressing
Key Uncertainties in Remote Canadian Environments) that contribute to our
fundamental understanding of Arctic aerosol particles as they relate to
climate forcing. The overall goal of NETCARE research has been to use an
interdisciplinary approach encompassing extensive field observations and a
range of chemical transport, earth system, and biogeochemical models. Several
major findings and advances have emerged from NETCARE since its formation in
2013. (1) Unexpectedly high summertime dimethyl sulfide (DMS) levels were
identified in ocean water (up to 75 nM) and the overlying atmosphere (up to
1 ppbv) in the Canadian Arctic Archipelago (CAA). Furthermore, melt ponds,
which are widely prevalent, were identified as an important DMS source (with
DMS concentrations of up to 6 nM and a potential contribution to atmospheric
DMS of 20 % in the study area). (2) Evidence of widespread particle
nucleation and growth in the marine boundary layer was found in the CAA in
the summertime, with these events observed on 41 % of days in a 2016
cruise. As well, at Alert, Nunavut, particles that are newly formed and grown
under conditions of minimal anthropogenic influence during the months of July
and August are estimated to contribute 20 % to 80 % of the 30–50 nm
particle number density. DMS-oxidation-driven nucleation is facilitated by
the presence of atmospheric ammonia arising from seabird-colony emissions,
and potentially also from coastal regions, tundra, and biomass burning. Via
accumulation of secondary organic aerosol (SOA), a significant fraction of the new
particles grow to sizes that are active in cloud droplet formation. Although
the gaseous precursors to Arctic marine SOA remain poorly defined, the
measured levels of common continental SOA precursors (isoprene and
monoterpenes) were low, whereas elevated mixing ratios of oxygenated volatile
organic compounds (OVOCs) were inferred to arise via processes involving the
sea surface microlayer. (3) The variability in the vertical distribution of
black carbon (BC) under both springtime Arctic haze and more pristine
summertime aerosol conditions was observed. Measured particle size
distributions and mixing states were used to constrain, for the first time,
calculations of aerosol–climate interactions under Arctic conditions.
Aircraft- and ground-based measurements were used to better establish the BC
source regions that supply the Arctic via long-range transport mechanisms,
with evidence for a dominant springtime contribution from eastern and
southern Asia to the middle troposphere, and a major contribution from
northern Asia to the surface. (4) Measurements of ice nucleating particles
(INPs) in the Arctic indicate that a major source of these particles is
mineral dust, likely derived from local sources in the summer and long-range
transport in the spring. In addition, INPs are abundant in the sea surface
microlayer in the Arctic, and possibly play a role in ice nucleation in the
atmosphere when mineral dust concentrations are low. (5) Amongst multiple
aerosol components, BC was observed to have the smallest effective deposition
velocities to high Arctic snow (0.03 cm s−1).

Rapid changes in the Arctic environment including rising temperatures, melting sea ice,
elongated warm seasons, and changing aerosol and trace gas long-range transport patterns
(IPCC, 2013) are driving a growing interest in developing a better understanding of the
processes that control Arctic climate. Furthermore, because high-latitude climate change
is a bellwether for change on a global scale, it is particularly important to understand
the feedbacks that lead to amplification of Arctic warming (Serreze and Barry, 2011).

This article discusses key discoveries that have been made in climate-related
Arctic aerosol research by the NETCARE (Network on Climate and Aerosols:
Addressing Key Uncertainties in Remote Canadian Environments) research
network. Formed in 2013, NETCARE consists of Canadian academic and government
researchers along with international collaborators. Given the highly diverse
nature of inter-related earth system processes that couple within the Arctic
environment, the network is necessarily interdisciplinary, consisting of
climate and air quality modellers, atmospheric chemists, aerosol and cloud
physicists, biological and chemical oceanographers, biogeochemists, and
remote sensing experts. Over the past 6 years, the network has conducted a
set of field campaigns and modelling projects focused on the sources and loss
mechanisms of atmospheric particles, their chemical and optical
characteristics, and their role in climate. The field studies were conducted
using a variety of platforms including the Alfred Wegener Institute's
Polar 6 aircraft (Herber et al., 2008), the research icebreaker
Canadian Coast Guard Ship (CCGS) Amundsen, and the Dr. Neil Trivett
Global Atmosphere Watch Observatory at Alert, Nunavut (hereafter, Alert).
Table 1 and Fig. 1 present the locations and dates of the field studies. The
modelling studies used the Canadian Atmospheric Global Climate
Model (CanAM; von Salzen et al., 2013), the GEOS-Chem chemical transport
model with associated microphysics module TOMAS (Croft et al., 2016b),
Environment and Climate Change Canada's GEM-MACH chemical transport model
(Moran et al., 2010), coupled
ice–ocean biogeochemistry models in 1-D and 3-D configurations (Hayashida et
al., 2018; Mortenson et al., 2018), and the Lagrangian particle dispersion
model FLEXPART (Stohl et al., 2005). The overall goals of the network have
been to study the nature of fundamental biogeochemical and physical processes
that connect aerosol to climate in environments that vary from pristine to
polluted, such as those found in the Arctic, in order to use this new
understanding to improve the accuracy of the different modelling approaches
used to simulate climate in these environments.

Figure 1Map of the Arctic indicating NETCARE field work locations, including the
ground station (Alert), CCGS Amundsen ship tracks in the summers of
2014 and 2016, and Polar 6 aircraft flights in summer 2014 (based
out of Resolute Bay) and in spring 2015 (based out of Longyearbyen, Alert,
Eureka, and Inuvik).

The network's output is documented through a special issue across three
journals: Atmospheric Chemistry and Physics;
Biogeosciences; and Atmospheric Measurement Techniques
(https://www.atmos-chem-phys.net/special_issue835.html; Bopp et al.,
2014), of which this article is a part. NETCARE has also produced a number of
publications in other journals. All of the NETCARE atmospheric measurements
are in a publicly available archive at https://open.canada.ca (last
access: 16 February 2019). The specific goal of this overview paper is to
synthesize the results from NETCARE and to act as a gateway into the more
detailed results described within the special issue and elsewhere.

Written for a scientist interested in the fields of Arctic climate,
atmospheric chemistry, and biogeochemistry, this article starts with a
background on Arctic aerosol that is not focused on NETCARE results
(Sect. 2). For additional background information, the reader is referred to
Quinn et al. (2006, 2008), Law and Stohl (2007), and Willis et al. (2018).
The article then presents new insights into the three topics around which
NETCARE was structured: marine processes and the Arctic atmosphere (Sect. 3);
the sources, sinks, and properties of Arctic aerosol (Sect. 4); and ice
nucleating particles (INPs; Sect. 5). Each of these sections stands alone, so
that the interested reader can focus their attention on a specific subject.
However, there are clear connections between the different topics. For
example, Sect. 3 (Marine processes and the summertime Arctic atmosphere) is
motivated by the increasing marine impact that is arising as sea ice melts
and focuses on new NETCARE Arctic measurements of dimethyl sulfide (DMS),
ammonia, and oxygenated volatile organic carbon species. The oceans are an
important source of such reactive gases to the atmosphere, leading to direct
impacts on aerosol particles and ultimately on climate. Those connections are
made in Sect. 4 (Arctic aerosol: sources, sinks, and properties), which
presents insights gleaned for the summertime environment, when these marine
emissions can lead to new particle formation and growth, and discusses the
impacts of this aerosol on clouds. Section 4 also presents results from the
Arctic haze springtime period, where the emphasis is on the sources of
particles, their optical properties, and the potential for direct radiative
forcing. Section 5 (Ice nucleating particles) addresses the select fraction
of atmospheric particles that nucleate ice crystals. Section 6 concludes the
article by discussing remaining research uncertainties and future priorities.

Over the last half century, our knowledge of Arctic aerosol and its role in
climate has advanced from almost nothing to a clear understanding of its
importance, although important questions remain regarding mechanistic
details. This short section of the paper presents a comprehensive
description of the field, leaving the recent NETCARE results for later
sections.

Following early observations of visibility-reducing haze particles in the
spring Arctic atmosphere (Greenaway, 1950), study of
Arctic haze began in earnest in the 1970s (Holmgren
et al., 1974; Rahn and Heidam, 1981). Investigations intensified through the
1980s, with observations (ground-based and airborne) and meteorological
analyses indicating that haze particles were transported from mid-latitude
pollution sources, often in layers that reached up to the tropopause, and
that their concentrations increased in winter and spring due to efficient
meridional transport and low rates of wet deposition (Barrie,
1986; Barrie and Hoff, 1985; Brock et al., 1989; Leaitch et al., 1989; Radke
et al., 1984; Schnell and Raatz, 1984; Shaw, 1982).

From the early studies of Arctic haze arose the concept of the Arctic atmosphere as a
dome of cold air that regulates transport of polluted air from southerly latitudes
(Barrie, 1986). The polar front extends in the winter to include more southerly
industrial emissions that can be transported into the high Arctic, and the front retreats
in the summer to inhibit transport from mid-latitude sources. Figure 2 shows an example
of identification of the polar dome in spring 2015 through measurements conducted during
the NETCARE aircraft campaign. Pollution transport into the Arctic may also be influenced
by the North Atlantic Oscillation (Duncan and Bey, 2004; Eckhardt et al., 2003). Arctic
haze originates from Eurasia, Siberia, southeast Asia and North America, with Eurasia as
the dominant source region at lower altitudes and contributions from south and central
Asian sources dominating at higher altitudes (Fisher et al., 2011; Qi et al., 2017;
Sharma et al., 2013; Stohl, 2006). Sea salt contributes to the haze due to the
combination of stronger winds and reduced wet deposition in the winter and spring (Huang
and Jaeglé, 2017; Leaitch et al., 2018) and frost flowers may contribute some marine
salt (Shaw et al., 2010). Snowpack exchange is a potential springtime source of organic
precursors (McNeill et al., 2012), while stratospheric contributions appear to be small
(Leaitch and Isaac, 1991; Stohl, 2006).

Figure 2The potential temperature (Θ) distribution binned in steps of 1∘
latitude and 20 hPa pressure. Θ was calculated from the temperature
and pressure measurements on board the Polar 6 aircraft during the
NETCARE 2015 springtime campaign. Minimum potential temperatures of less than
270 K were only observed in the high Arctic lower troposphere, representing
very cold air masses that isolate this area from mid-latitudinal influence.
The polar dome is formed by the sloping isentropes which can be identified
from the NETCARE measurements. Figure from Bozem et al. (2019).

However, many components of Arctic haze (e.g. sulfate; organic matter, OM; sea salt)
help to cool the Arctic by scattering light back to space (Schmeisser et al.,
2018) and by modifying the microphysics of liquid clouds to enhance shortwave
cooling (Garrett and Zhao, 2006; Lubin and Vogelmann, 2006; Zamora et al.,
2017; Zhao and Garrett, 2015). During winter and spring, sulfuric acid in
Arctic haze particles may reduce their effectiveness as INPs, leading to
larger crystals that precipitate more easily. As a result, there may be an
increase in the dehydration rate of the atmosphere and a corresponding
reduction in longwave forcing (Blanchet and Girard, 1994; Curry and Herman,
1985). At cirrus temperatures, dust, ammonium sulfate, and sea salt may also
increase cloud albedo by increasing ice crystal concentrations (Abbatt et
al., 2006; Sassen et al., 2003; Wagner et al., 2018).

Observed and simulated seasonal cycles of BC and sulfate typically show a maximum in
near-surface concentrations in March or April (Barrie and Hoff, 1985; Eckhardt et al.,
2015; Garrett et al., 2010; Sharma et al., 2006) and clean conditions in the summertime.
Natural emissions of BC from vegetation fires are considerable in late spring to early
summer in the Arctic and at mid-latitudes (Mahmood et al., 2016). Production of sulfate
aerosol is more efficient in the warm than the cold seasons (Mahmood et al., 2018; Tesdal
et al., 2015). The decline in Arctic haze after its peak in early spring and the approach
to the summertime pristine conditions are largely related to changes in transport as the
polar front moves northward and aerosol scavenging rather than a reduction in aerosol
production. Wet deposition associated with transport across the retracted polar front,
frequent low-intensity precipitation, and longer residence times within the polar dome
keep the summertime near-surface Arctic nearly free of anthropogenic aerosol (Barrie,
1986; Stohl, 2006; Garrett et al., 2010; Browse et al., 2012). However, at higher
altitudes up to 8 km, long-range transport from mid-latitude pollution into the Arctic
was also observed in summer (Schmale et al., 2011). Marine sources have a strong
influence on the Arctic summer aerosol near the surface and possibly aloft (Dall'Osto et
al., 2017; Korhonen et al., 2008b; Stohl, 2006).

Overall, the net effect of anthropogenic aerosols has been to cool the Arctic (Fyfe et al., 2013; Najafi et al., 2015),
and Navarro et al. (2016) showed that
reductions in Arctic haze have contributed to the sharp increase in the rate
of Arctic warming since 1990. Mitigation of BC emissions may help to slow
Arctic warming so long as cooling components are not simultaneously mitigated (Kopp
and Mauzerall, 2010; Sand et al., 2013; Shindell and Faluvegi, 2009).

As seen from this brief overview, understanding natural aerosol processes in
addition to anthropogenic aerosol sources is vital for climate studies, as
anthropogenic aerosol forcing is measured against the natural component (Carslaw et al., 2013; Megaw
and Flyger, 1973). For example, in the winter and spring, sea salt aerosol
may play an important climate role (Kirpes et al.,
2018). At the start of NETCARE, detailed knowledge of natural particle
sources and their impacts on clouds in the nearly pristine summer was
incomplete, and it became a major focus of the network's research
activities.

3.1 Rationale and research questions

In remote marine atmospheres such as the summertime Arctic, assessing the impact of
natural marine biogenic aerosol (MBA) sources on cloud formation is pivotal to accurately
estimating climate forcing (Carslaw et al., 2013; Charlson et al., 1987). While a variety
of organic compounds, such as marine microgels, may be relevant primary MBA sources in
the Arctic (Leck and Bigg, 2005; Orellana et al., 2011), DMS-derived sulfate is thought
to be a key precursor to secondary marine aerosol mass over biologically productive
regions (Ghahremaninezhad et al., 2016; Leaitch et al., 2013; McCoy et al., 2015; Park et
al., 2017). The production of DMS and other organic compounds in polar regions is linked
to the productivity of microalgae, as well as to the dynamics and the structure of
pelagic (oceanic) and sympagic (ice-associated) microbial food webs (Gabric et al., 2017;
Levasseur, 2013; Simó, 2001; Stefels et al., 2007). Peaks in the DMS proxy MSA have
been observed in association with bursts of phytoplankton productivity in the high Arctic
(Becagli et al., 2016). As well, atmospheric DMS mixing ratios in the marine boundary
layer have been shown to transiently peak during the phytoplankton growth period from May
to September (Park et al., 2013, 2018). Particle nucleation and growth events have been
observed even at moderate levels of atmospheric and oceanic DMS in the high Canadian
Arctic (Chang et al., 2011b; Rempillo et al., 2011).

Despite these compelling indications of the key role played by marine biogenic DMS in
contributing to sulfate aerosols (Rempillo et al., 2011), measurements of seawater and
sea-ice DMS during the biologically productive summer months (June to August) that
coincide with clean aerosol time periods are still scarce (Jarníková et al.,
2018; Levasseur, 2013). The paucity of DMS measurements in ice-associated habitats, such
as under the sea ice, in melt ponds atop the ice, or directly at the Arctic sea-ice
margin, is even greater (Levasseur, 2013). Sea ice not only acts to modulate gaseous
exchange but also hosts active microorganisms (Gradinger, 2009), making it a fundamental
driver of various MBA precursors, including DMS (Arrigo, 2014; Gabric et al., 2017;
Korhonen et al., 2008b). Our understanding of the processes that control other key gases
that can lead to aerosol formation in marine environments, including ammonia and volatile
organic compounds (VOCs), is particularly weak. There have been very few measurements of
their Arctic abundance in the past and we have a poor understanding of their sources. In
this context, NETCARE targeted the spatio-temporal variability in DMS and the underlying
ecosystemic mechanisms controlling its abundance in the eastern Canadian Arctic (Canadian
Arctic Archipelago, henceforth CAA, and northern Baffin Bay), along with the atmospheric
abundances and sources of other key gases.

3.2 DMS production in oceanic and ice-associated environments

The two NETCARE summer campaigns (July–August 2014 and 2016; see Fig. 1 and Table 1)
revealed high open-water concentrations of DMS (interquartile range of
5.1–10.9 nmol L−1, maximum 75 nmol L−1) in the eastern Canadian Arctic.
Previous pan-Arctic measurements had an interquartile range of 0.9–5.9 nmol L−1
and a maximum of 26 nmol L−1. These results challenged the representativeness of
measurements conducted during previous cruises in the Eastern Canadian Arctic in late
summer and early fall (Luce et al., 2011; Motard-Côté et al., 2012) by showing
that average summer surface DMS concentrations in this part of the Arctic were at least
2-fold higher than measurements conducted later in the season. The range of seawater DMS
concentrations measured in the CAA during the NETCARE expeditions in 2014 (Fig. 3) and
2016 is comparable to those observed in the same area and season in 2015 by
Jarníková et al. (2018), who found the highest DMS concentrations in association
with localized peaks of chlorophyll a, a proxy of phytoplankton biomass. Combining
oceanic and atmospheric NETCARE data sets provided further evidence that marine DMS
hotspots were associated with high atmospheric DMS (Mungall et al., 2016). As described
in Sect. 4, connections were also found between localized regions of high oceanic
biological activity and new particle formation and growth events (Collins et al., 2017;
Mungall et al., 2016) that may be partly caused by DMS and organic emissions. These new
observations lend strong support to the hypothesis that local Arctic DMS sources are
responsible for the summertime peaks in MSA measured at Alert (Leaitch et al., 2013;
Sharma et al., 2012).

Figure 3(a) Concentrations of DMS (nmol L−1) in ice-free waters as a
function of depth (m) with moving average line (all data, n=208). The grey dotted
line represents the average surface mixed layer depth (Zm=21 m) estimated
as the depth at which the gradient in density between two successive depths was
>0.03 kg m3. (b) Concentrations of DMS (nmol L−1) in melt ponds
(n=9) atop first-year sea ice (Gourdal et al., 2018) and in under-ice waters (n=3). All data are from the 2014 NETCARE cruise on board CCGS Amundsen.

Novel measurements made during NETCARE also substantiated the potentially
important role played by melt ponds. An in-depth study of nine melt ponds
revealed that brackish melt ponds over first-year sea ice (FYI) may have DMS
concentrations ranging from 3 to 6 nmol L−1 (Fig. 3) with an average of
3.7 nmol L−1 (Gourdal et al., 2018). These
concentrations are higher than the area-weighted mean of ca. 2.4 nmol L−1 derived from the global climatology of Lana et al. (2011), bringing
support to the suggestion that melt ponds may represent a significant source
of DMS in the Arctic. A search for the underlying mechanisms associated with
the presence of DMS in these melt ponds revealed that intrusions of seawater
through permeable sea ice is a key physical process allowing their
colonization by DMS-producing marine protists (Gourdal et
al., 2018). Considering that the areal coverage of melt ponds may extend up
to 90 % over Arctic FYI (Rösel et
al., 2012) these results shed light on a previously overlooked source of DMS
to the atmosphere and further call for a re-evaluation of the emissions from
these regions within climatologies that currently assume the absence of DMS
fluxes above ice-covered waters (Lana et al., 2011). In
a simulation exercise, melt ponds were found to contribute an average of 20 % (and up to 100 %) of the atmospheric DMS over and near ice-covered
regions of the Arctic during the melt season (Mungall et
al., 2016).

While marginal ice zones (MIZs) and various ice-edge systems have long been recognized
for their teeming biological activity (Mundy et al., 2009; Perrette et al., 2011) and
potential for heightened DMS production (Galí and Simó, 2010; Levasseur, 2013;
Matrai and Vernet, 1997), they remain surprisingly under-documented for their specific
role in MBA production in the eastern Canadian Arctic during summer. Two distinct MIZs
explored during the summer of 2014 revealed highly contrasting DMS dynamics, suggesting
that whether the sea ice is FYI or multi-year ice (MYI) is of paramount importance in
shaping marine food webs and the net production of DMS in the water exiting the ice pack.
Contrasting DMS dynamics between FYI and MYI systems were likely linked to differences in
light penetration through the ice pack and its availability to primary producers in the
waters just below the ice. At the MYI edge in Kennedy Channel (ca. 81∘ N), DMS
concentrations were very low at the ice edge and increased progressively over several
kilometres as the water flowed away from the ice pack, suggesting that time out from
under the ice was required for development of a phytoplankton bloom and the concomitant
production of DMS. However, at a FYI edge in the CAA (ca. 74∘ N), DMS
concentrations were already high under the ponded sea ice (Fig. 3) due to the presence of
an under-ice bloom. Consequently, the surface waters exiting the ponded FYI displayed
high DMS even at the very edge of the ice pack. The elevated levels of DMS persisted for
several kilometres away from the ice edge. Thus, beyond the direct contributions melt
ponds make to DMS fluxes, results from the NETCARE campaigns suggest that melt ponds play
an additional role in DMS dynamics by promoting the earlier onset of under-ice
phytoplankton blooms and DMS production (Lizotte et al., 2019). Taken together, these
observations reveal the potential for high DMS emissions to the atmosphere immediately
upon the cracking, opening, or melting of ponded FYI without the prerequisite of an
ice-free period to initiate a phytoplankton bloom and potential accumulation of DMS in
surface waters.

High levels of DMS have previously been associated with aerosol formation and growth in
the CAA (Chang et al., 2011b; Park et al., 2017; Rempillo et al., 2011). As part of
NETCARE, new atmospheric measurements of DMS were performed from both the
Polar 6 aircraft and the CCGS Amundsen icebreaker. Mean DMS mixing
ratios in the Arctic lower free troposphere in April 2015 were found to be unexpectedly
high (average 116±8 ppt) relative to those from the July 2014 campaign (20±6 ppt) (Ghahremaninezhad et al., 2017). The springtime levels likely reflect long-range
transport from more southerly open-ocean regions. In the summertime, the boundary layer
mixing ratios were at times much higher than they were in the spring in both 2014
(Mungall et al., 2016) and 2016 (unpublished results), reflecting nearby marine sources.
For example, high atmospheric DMS concentrations (up to 1800 ppt, median 144 ppt) were
found within the boundary layer from ship-based grab samples collected in July and August
2016. For a similar period and location in 2014, these values were up to 1100 ppt
(median 186 ppt; Mungall et al., 2016). Evidence for atmospheric DMS was the widespread
prevalence of biogenic DMS oxidation products in the marine boundary layer
(Ghahremaninezhad et al., 2016).

VOCs were measured in the marine atmosphere during the 2014 CCGS Amundsen
cruise. Isoprene and monoterpene levels were frequently below detection limit, but
occasionally reached as high as 15 ppt (Mungall et al., 2016). Oxygenated volatile
organic compounds (OVOCs) were also measured, using an instrument that is especially
sensitive to organic acids. High levels of formic acid (up to 4 ppb) and isocyanic acid
(up to 80 ppt) were strongly correlated with a suite of
C4–C11 oxo-acids (Mungall
et al., 2017). Using positive matrix factorization, these OVOCs, which were elevated in
regions where the ocean had high dissolved organic carbon (DOC) content, were interpreted
as originating from an ocean source (Fig. 4). Production at the sea surface microlayer
was invoked as an explanation, because compounds like formic acid are sufficiently
soluble that they should not escape from the bulk ocean into the atmosphere. Rather,
these species must arise either through photochemistry or heterogeneous oxidation
proceeding in the sea surface microlayer, or by gas-phase atmospheric oxidation of
components volatilized from the microlayer. Although the OVOC molecules measured by
Mungall et al. (2017) were too volatile to participate in formation of Arctic marine
secondary organic aerosol (MSOA) themselves, similar processes that form larger, less
volatile molecules could contribute to aerosol growth. We note that there was a weak
positive correlation between total aerosol volume and the levels of OVOCs observed,
indicating a potential link between the processes forming OVOCs and aerosol growth.
Formation of Arctic MSOA and its role in new particle growth in the Arctic is discussed
further in Sect. 4.2. (Note that in this work we use the term Arctic MSOA to refer to the
organic aerosol formed in the atmosphere from marine biogenic emissions. We note that the
chemical character of Arctic MSOA is not necessarily the same as that in other marine
environments. For example, different biogenic precursors may be present elsewhere, and
the SOA that forms from shipping emissions will have very different properties and
composition.)

Figure 4A large suite of oxygenated VOCs (OVOCs) were measured on the CCGS
Amundsen during the 2014 cruise in the high Canadian Arctic. A factor analysis
of the full time-dependent data set yielded an “Ocean factor” of small organic acids
whose intensity correlated with the DOC levels in the ocean (a) and with time of
day and hence downwelling radiation (b). See text for additional discussion.
Figures from Mungall et al. (2017).

NETCARE provided the opportunity to make some of the first observations of ammonia in the
Arctic atmosphere. Previous measurements of atmospheric ammonia over the Norwegian Sea
and Arctic Ocean during the summer ranged between the detection limit (35 ppt) and
400 ppt (Johnson et al., 2008). Simultaneous measurements of sea surface ammonium
(NHx) during these previous studies ranged between 29 and 616 nM,
leading to ammonia compensation points that were below the ambient concentrations,
suggesting that the ocean could not act as a source of ammonia to the atmosphere. During
the 2014 NETCARE campaign, hourly atmospheric ammonia measurements in the CAA marine
boundary layer ranged between 40 and 870 ppt (Wentworth et al., 2016). Simultaneous
measurements of NHx at the sea surface and in melt ponds confirmed that
these reservoirs could not act as sources of ammonia to the atmosphere. Boreal fires
contributed to elevated atmospheric NH3 in the CAA during 2014 (Lutsch et al.,
2016), but could not fully explain the spatial and temporal extent of the elevated
NH3 mixing ratios. The inclusion of NH3 emissions from migratory
seabird colonies in model simulations brought predicted NH3 values into much
better agreement with observations (Wentworth et al., 2016) and strongly influenced
modelled new particle formation (Croft et al., 2016a). In 2016, observations were again
made from the CCGS Amundsen but at a higher time resolution, as well as at the
Alert field site, both from mid-June to mid-July (Murphy et al., 2019). The ranges of
hourly average NH3 values measured from the ship in 2016 (up to 1150 ppt;
median 125 ppt) and at Alert (up to 720 ppt; median 234 ppt) were similar to the
observations in 2014. Limited measurements of the tundra soil emission potential at the
Alert site indicated that under the unusually high temperatures experienced at Alert in
July 2016, the tundra could act as a source of ammonia to the atmosphere. Overall, the
bidirectional exchange of ammonia between the atmosphere and the land–ocean surface is
important to include in chemical transport models. The impact of ammonia on aerosol
formation in the summertime Arctic, with associated climate impacts, is discussed below
in Sect. 4.2.

3.4 Connecting the ocean, sea ice and the atmosphere through DMS modelling

Prior to the NETCARE field campaigns, the existing un-extrapolated DMS
climatology, averaged over the most productive time of the year (months of
July and August), clearly demonstrated the scarcity of surface ocean DMS
measurements in the Arctic (Lana et al., 2011). The updated Lana DMS
climatology and its precursor (Kettle et al., 1999) have long represented
useful tools for oceanic model validation (e.g. Le Clainche et al., 2010;
Tesdal et al., 2016; Kim et al., 2017) and lack of data over the Canadian
Polar shelf and the Baffin Bay area challenged the representativeness of the
standard (extrapolated) version of this climatology for these specific
regions (Fig. 5c). Observations gathered through NETCARE field campaigns
(Fig. 5b) significantly enhanced coverage in these regions.

As part of NETCARE, a new process-based sea-ice–ocean biogeochemical model representing
ecosystems in both the sea ice and water column of the marine Arctic was developed. The
model was initially developed in a one-dimensional (1-D) configuration (Mortenson et al.,
2017). Subsequently, sulfur and inorganic carbon cycling were developed and implemented
into the model (Hayashida et al., 2017; Mortenson et al., 2018). The simulated Arctic
sea-ice ecosystem and sulfur cycle were next incorporated into a three-dimensional (3-D)
regional configuration (Hayashida, 2018; Hayashida et al., 2018). This model advances
previous Arctic-focused DMS model studies (Elliott et al., 2012; Jodwalis et al., 2000)
in that many of the parameters concerning the DMS production are derived from recent
field observations in the Arctic, enabling quantification of the relative contributions
of ice algae and phytoplankton to DMS production and emissions. The 1-D simulations
indicated a notable contribution of ice algae: an 18 % enhancement of DMS
concentrations under the ice and a 20 %–26 % enhancement of sea–air DMS fluxes
during the melt period for Resolute Passage (Hayashida et al., 2017). Also in the
vicinity of ice margins, simulated spikes in sea–air fluxes of DMS originating from
bottom and under-ice production by algae were comparable to some of the local maxima in
the summertime flux estimated for ice-free waters in the Arctic.

The data obtained during the two NETCARE ship campaigns, together with data
previously available in the PMEL sea surface database
(https://saga.pmel.noaa.gov/dms/, last access: 16 February 2019), were
used to develop a new satellite-based model allowing the estimation of DMS at
the global and regional scales (Galí et al., 2018). As can be seen in
Fig. 5d, the satellite-based model provides a DMS map with an unprecedented
spatial resolution (8 days, 28 km×28 km pixels). The DMS
climatology based on the 3-D process-based model simulation shows a range
similar to the Lana et al. (2011) climatology, but higher spatial
variability, in line with the satellite-based climatology (Fig. 5e). Together
with the remote sensing approach, the numerical model is being used to help
interpret the new NETCARE DMS data set, as well as to investigate longer-term
and larger-scale variability, such as impacts of sea-ice reduction on DMS
production (Hayashida, 2018).

Under future global warming conditions, sea-ice extent is expected to decline
significantly, affecting the temporal and spatial evolution of ice algae and under-ice
and open-water phytoplankton blooms. This may lead to changes in oceanic DMS emissions,
although the sign and magnitude of the change is highly uncertain. Using the satellite
approach mentioned above, Galí et al. (2019) showed that DMS emission has increased
at a rate of about 30 % decade−1 during the last 2 decades, accompanied by
large inter-annual changes linked to variable ice retreat patterns. They also estimated a
2- to 3-fold increase in DMS emission in response to complete sea-ice loss in summer.

To estimate the sensitivity of Arctic aerosols and radiative forcing to surface seawater
concentrations of DMS in the Arctic, simulations with different specified surface
seawater DMS concentrations and spatial distributions in the Arctic were performed for
future sea-ice conditions using the Canadian Atmospheric Global Climate Model (CanAM4.3).
For all of the specified surface seawater DMS conditions in the model, simulated Arctic
sulfate aerosol amounts respond only weakly to a reduction in sea-ice extent owing to
increases in precipitation and aerosol wet deposition associated with the receding ice
and increased open water (Mahmood et al., 2018). However, nucleation rates for sulfate
aerosol respond significantly to reductions in sea-ice extent, which leads to a
strengthening of cloud radiative forcing in the future. Furthermore, the simulated
response of the mean cloud radiative forcing in the Arctic is approximately proportional
to the mean surface seawater DMS concentration in the Arctic. Thus potential future
changes in sea-ice extent may result in a negative climate feedback of DMS on radiative
forcing in the Arctic, as suggested by Charlson et al. (1987).

4.1 Rationale and research questions

The overall motivation of Arctic summertime research is to determine how the atmosphere
will respond to melting sea ice, as an ocean that was largely covered by sea ice through
much of the summer will potentially be ice free in summer by mid-century (AMAP, 2017;
Comiso, 2011; Gregory et al., 2002; Holland et al., 2006). Given the evolution of the
summertime Arctic Ocean from a bright ice cap to a dark ocean that can readily absorb
solar radiation, it is of particular importance to understand factors controlling the
overhead aerosol and cloud that could mediate the positive radiative feedback of
declining sea ice. Precipitation associated with low clouds and fogs is common in the
summertime (Browse et al., 2014). Wet deposition is a highly efficient aerosol removal
mechanism, giving rise to a clean boundary layer in which new particles may be formed or
into which they may be input. In these clean boundary layers, increases in the numbers of
particles acting as cloud condensation nuclei (CCN) may increase longwave warming by
clouds if the absolute concentrations of CCN are sufficiently low (Mauritsen et al.,
2011); otherwise, increases in CCN concentrations lead to enhanced shortwave cooling. In
this context, it is important to better understand the processes that give rise to new
particle formation and growth to CCN sizes, and the associated impacts on clouds. For
example, how do the emissions of biogenic gases described in Sect. 3 affect new particle
formation and growth in such environments, and what is their importance relative to
anthropogenic inputs from local shipping and long-range transport?

In contrast, the springtime atmosphere, with its associated Arctic haze, has
been better studied than the summertime atmosphere. The results from high
profile campaigns such as ISDAC (Indirect and Semi-Direct Aerosol Campaign,
https://campaign.arm.gov/isdac/, last access: 16 February 2019), ARCTAS
(Arctic Research of the Composition of the Troposphere from Aircraft and
Satellites, https://www.nasa.gov/mission_pages/arctas/, last access: 16
February 2019), and ARCPAC (Aerosol, Radiation, and Cloud Processes affecting
Arctic Climate, https://www.esrl.noaa.gov/csd/projects/arcpac/, last
access: 16 February 2019) have emphasized the importance of long-range
transport (see also the POLARCAT project; Polar Study using Aircraft, Remote
Sensing, Surface Measurements and Models, of Climate, Chemistry, Aerosols,
and Transport, https://www.atmos-chem-phys.net/special_issue182.html;
Stohl et al., 2009). However, many questions remain. Taking BC-containing
aerosol as an example, we can ask the following questions. What is the
relative importance of sources in Europe and different Asian regions (Jiao
and Flanner, 2016), and how does the relative importance of different source
regions vary vertically from the surface to higher altitudes? To what degree
can specific source regions be identified? How important are local sources,
such as from Arctic shipping or gas flaring? How will the direct effect of
light-absorbing particles be impacted by their mixing state, that is, by the
degree to which they are internally or externally mixed with other components
of the pollution plume? More generally, the composition of the air masses
throughout the Arctic needs to be better evaluated vertically to aid in the
identification of long-range transport sources, to help establish whether
chemical transformations occur during transit and descent within the Arctic
air mass, and to ultimately better estimate climate impacts.

Figure 6Aerosol size distributions from Alert and Zeppelin Arctic field
stations. The pronounced accumulation mode in the winter and spring is
characteristic of Arctic haze. The mode of Aitken particles is a common
feature of the Arctic summertime atmosphere. Figure from Croft et
al. (2016b).

Lastly, the deposition rates of aerosol constituents need to be measured to
better constrain models. Ideally, both wet and dry deposition rates would be
individually evaluated throughout the year, to map out the transition from a
system dominated by the relatively slow loss with ice cloud scavenging
versus the more efficient removal via warm clouds and fogs.

4.2 Summertime aerosol: particle formation and growth

As described in Sect. 2, a pronounced Aitken mode in the aerosol size
distribution is a common feature during the Arctic summertime, as
demonstrated by Croft et al. (2016b), who identified this feature in
long-term monitoring data sets from both the Alert and Zeppelin ground
stations (Fig. 6). One of the major findings from NETCARE is the widespread
prevalence of 5–50 nm ultrafine particles in the summertime Canadian Arctic
(Burkart et al., 2017b; Collins et al., 2017; Willis et al., 2016, 2017) and
their ability to activate as CCN (Burkart et al., 2017a; Chaubey et al.,
2019). While previous ship-based measurements in similar regions in late
summer and fall had demonstrated new particle formation and growth events,
their frequency was low. For example, in the fall period of late August to
the end of September 2008, only three such events were observed over a 5-week
observation period, whereas no events were observed at all in October 2007
(Chang et al., 2011b). By comparison, NETCARE measurements in mid-July to
mid-August 2016 observed enhancements in 5–50 nm particles 41 % of the
time in a spatially heterogeneous manner (Collins et al., 2017).
Characterization of the summertime increase in particles is provided in Fig. 7, wherein the number of particles
between 15 and 30 nm (N15–N30) is highly enhanced at Alert in July and
August, before rapidly declining in September (see the Supplement for
details). As discussed in the Supplement, natural sources are estimated to
contribute 20 %–80 % of the 30–50 nm particles during July and
August. NETCARE aircraft measurements in July 2014 also demonstrated the
spatial heterogeneity of 5–50 nm particle numbers in the inversion layer,
with the highest concentrations observed over marine and cloudy regions and
little detectable enhancement over ice-covered areas (Burkart et al., 2017b).
These aircraft measurements also indicate that the numbers of these tiny
particles in the free troposphere are spatially homogeneous and considerably
lower than those measured in the inversion layer, indicative of a boundary
layer source.

Figure 7The changing composition and size distributions of aerosol in the
high Arctic, see the Supplement for details. (a) Monthly average
number concentrations for the indicated size ranges for measurements at
Alert. (b) Estimated increases in particles in the indicated size
intervals for June–September, inclusive, and monthly average values of
OM/nss-SO42- (non-sea-salt sulfate) based on weekly filter
samples. The data presented here are from April 2012 to October 2014,
inclusive. The dashed lines in panel (a) represent an estimate of
number concentrations assuming no new particle formation. The number
concentration curves in panel (b) are the difference between the
solid and dashed curves in panel (a).

Figure 8Panel (a) illustrates that the number of CCN (at 0.6 %
supersaturation) measured by NETCARE in the summertime Arctic in 2014 is related to the
organic mass fraction of the particles measured by an aerosol mass spectrometer. Open
circles are all the data points. The closed, coloured circles represent the FLEXPART-WRF
predicted air mass residence time over open water in the boundary layer prior to the
measurement (see Willis et al., 2017, for details). Panel (b) plots the
H ∕ C vs. the O ∕ C ratios of submicron aerosol measured during the same
summertime 2014 campaign. The circles and triangles are low (<300 m) and high (>300 m) altitude points, respectively, and the colour is the MSA-to-sulfate ratio of the
aerosol. High ratios indicate large biogenic secondary impact. The convergence of points
with high ratios to an H ∕ C ratio close to 2 indicates a composition with
substantial hydrocarbon-like character, as indicated in red by the placements for common
molecules. Figures from Willis et al. (2017).

Significant growth of 5–50 nm particles to CCN sizes was clear from each
observational platform. At Alert (Fig. 7), the summertime enhancement in
particles between 15 and 30 nm (N15–N30) coincides with the increase in
particles in the 50 to 100 nm size range (N50–N100), which is also the size
of particle activation diameters observed in the field (see Sect. 4.3).
Interestingly, using Fourier Transform Infrared (FTIR) absorption of particulates
collected on filters, the ratio of aerosol organic material to sulfate was
also observed to increase during this time period, and the region of amide
functional groups indicates a contribution of organic components from
breakdown of seabird urea in guano (Leaitch et al., 2018). Likewise, a
particle growth episode was clearly observed over the ice-free Lancaster
Sound, in which the numbers of 5–50 nm particles and CCN increased in
concert with the measured organic content of the PM1 aerosol
(Willis et al., 2016). Across the entire aircraft campaign, the numbers of
CCN were most strongly enhanced above background levels when the air had
recently been at low altitude over open water (Fig. 8a), when the wind speeds
were low, and when the organic-to-sulfate ratio of the particles was high
(Willis et al., 2017). This marine influence is consistent with summertime
single-particle mass spectrometric measurements of trimethylamine-containing
particles in the marine boundary layer that were largely externally mixed
with sea-salt-containing particles (Fig. 9; Köllner et al., 2017).

The lack of a wind speed dependence and the observations of externally mixed particulate
trimethylamine suggests that secondary sources are important. Similarly, microphysical
models of growing particle size distributions could only be reconciled with observations
from the CCGS Amundsen icebreaker in northern Baffin Bay by invoking
partitioning of semi-volatile species to the freshly nucleated and pre-existing particles
(Burkart et al., 2017a). This stands in contrast to mid-latitude continental settings,
where the growth behaviour is best modelled by considerable condensation of low
volatility species such as sulfuric acid and highly oxygenated organic molecules. We
presume this semi-volatile material is organic in nature (i.e. Arctic MSOA).

Figure 9Single-particle mass spectrometry results from the NETCARE 2014 summer campaign,
where the detected particle fraction is plotted against the aerodynamic diameter of the
particle. The total number of particles detected in a specific size bin is plotted in
red. The classifications of particle types containing different species are: Na ∕ Cl
(dark blue), levoglucosan (grey), Na (green), elemental carbon (EC, black), and a
category of particles called “Secondary” that includes organics, potassium, sulfate,
trimethylamine, and MSA (light blue). Figure from Willis et al. (2017).

Natural emissions of ammonia are also important to new particle formation and growth.
Wentworth et al. (2016) used GEOS-Chem model simulations to interpret NETCARE ammonia
measurements (see Sect. 3.3) and found that migratory seabird colonies (emitting 36 Gg
NH3 between and May and September) were important sources of ammonia in the
summertime Arctic. In addition, transport of boreal wildfire smoke from lower latitudes
can also be an important, albeit episodic, contributor of ammonia to the summertime
Arctic troposphere (Croft et al., 2016a; Lutsch et al., 2016; Wentworth et al., 2016).
Croft et al. (2016a) further interpreted NETCARE observations using the GEOS-Chem-TOMAS
model to find that ammonia from seabird-colony guano is a key factor contributing to the
bursts of newly formed particles that are observed every summer at Alert (Fig. 10). In
addition, the FTIR absorption in the region of amide functional groups indicates a
contribution of organic components from the breakdown of seabird urea in guano. The
chemical transport model simulations indicate that the pan-Arctic seabird-influenced
particles can grow by sulfuric acid and organic vapour condensation to diameters
sufficiently large to enhance pan-Arctic cloud droplet number concentrations and effects on climate
in the clean Arctic summertime. Other natural ammonia sources within the same order of
magnitude as the seabird-colony emissions, including but not limited to episodic biomass
burning influences (Lutsch et al., 2016) and tundra emissions (Murphy et al., 2019),
could also contribute to these effects (Croft et al., 2018).

Figure 10Time series of measured and modelled numbers of particles 10 nm and
larger at Alert during 2011. Seabird ammonia is included in the blue curve
simulation but not in the red curve simulation. Measurements are in black.
Figure from Croft et al. (2016a). GCT represents
GEOS-Chem-TOMAS.

Determining the precursors to Arctic MSOA is of crucial importance. Aerosol mass
spectrometry measurements from the aircraft campaign in summer 2014 indicate that the
organic chemical character of this aerosol is distinctly different from that which arises
from oxidation of common continental precursors, such as isoprene or the monoterpenes
(Willis et al., 2017). The mass spectral signatures indicate molecules that instead have
substantial alkyl components, such as functionalized fatty acids (Fig. 8b). Long-chain
fatty acids can sometimes be a significant component of the sea surface microlayer
(Cunliffe et al., 2013). Croft et al. (2018) have shown that a steady flux of condensable
organic material from the ocean that oxidizes with a lifetime of a day is essential for
consistency between GEOS-Chem-TOMAS modelled aerosol size distributions and those
measured at Alert and from the CCGS Amundsen icebreaker. This evidence strongly
supports the importance of Arctic MSOA in setting the overall aerosol composition and
size in the summertime.

4.3 Summertime aerosol: impacts on liquid water clouds

Studies at mid-latitudes have routinely shown that the smallest particles that can serve
as nuclei for liquid cloud droplets are 80–120 nm in diameter (Hoppel et al., 1985;
Leaitch et al., 1986). The smaller Aitken particles, 20–80 nm in size, are commonly
considered to be too small to activate into cloud droplets. However, there are two
circumstances which enable Aitken particles to activate at cloud base: (1) rapid cooling
rates, generally associated with higher updraft speeds, increase cloud base
supersaturation; and (2) very low concentrations of larger particles (>100 nm), which
reduce water vapour uptake at cloud base, thereby increasing supersaturations. In the
second case, which is prominent in the Arctic during summer, modelling suggests that even
modest updrafts (20–50 cm s−1) lead to the activation as CCN of particles as
small as 40 nm (Korhonen et al., 2008b, a). This had never previously been verified by
observations and was a main focus of the NETCARE summertime flight campaign.

During the NETCARE flights conducted out of Resolute Bay in July 2014, number
size distributions of cloud droplets and aerosol particles measured in and
around clouds showed that 50 nm particles were routinely activated and that
particles as small as 20–30 nm were activated on a few occasions where
updraft speeds were likely higher (Leaitch et
al., 2016). These results substantiate the prediction made by Korhonen et
al. (2008b). However, Leaitch et
al. (2016) found
no evidence for an association of cloud liquid water content with aerosol
variations when droplet concentrations are less than about 10 cm−3,
which was proposed by Mauritsen et al. (2011)
as a means of aerosol-induced longwave warming. Modelling conducted as part
of NETCARE demonstrated the importance of this Aitken particle activation.
For example, as mentioned above, Croft et
al. (2016a) estimated the Arctic
summertime shortwave radiative forcing by the effects of natural seabird
ammonia emissions on these particles at −0.5 W m−2, highlighting the
importance of this natural aerosol for climate.

Lastly, experiments are in progress to evaluate the Single Column Model of
Arctic Boundary Layer Clouds (SCM-ABLC) and version 18 of the Canadian
Climate Centre's radiative transfer model with the cloud observations
conducted from Resolute Bay discussed above. The modelling work will attempt
to reproduce the observations and quantify the uncertainty in modelling the
radiative balance of low clouds and fog in the summertime Arctic.

4.4 Springtime aerosol: sources and vertical distribution

As discussed in Sect. 2, Arctic haze is a prominent feature in springtime, yet its
composition and sources remain uncertain. During the NETCARE 2015 aircraft campaign,
vertically resolved observations of trace gases and aerosol composition were made in the
high Arctic springtime, with six flights north of 80∘ N. Trace gas gradients
observed on these flights defined the polar dome (i.e. the region north of the Arctic
front) as north of 66–68.5∘ N and below potential temperatures of
283.5–287.5 K (Fig. 2; Bozem et al., 2019; Willis et al., 2019).

Model simulations of air mass history using FLEXPART indicate differences in transport
history as a function of potential temperature in the polar dome. Air masses at lower
potential temperature (lower altitude) spent long times (>10 days) in the polar dome,
while air masses at higher potential temperature (higher altitude) had entered the Arctic
more recently and were more recently influenced by lower latitude sources at the surface
(Willis et al., 2019). Model results indicate that descent of air masses from higher
potential temperatures influenced the lower polar dome on the timescale of 10 days.
Submicron aerosol composition varied systematically with model-predicted time spent in
the mid-to-lower polar dome (i.e. below 265 K): the sulfate fraction increased with time
below 265 K, while the NH4, OA, and BC fractions decreased significantly.
These phenomena could arise from a combination of three possible processes:
(1) systematic changes in source region with increasing potential temperature (Fisher et
al., 2011) that supply aerosol with systematically different compositions, (2) oxidation
of transported aerosol and sulfur dioxide over the long aerosol lifetime in the polar
dome and during transport, and (3) wet removal and cloud processing along the transport
path that may impact the composition of aerosol arriving in the polar dome.

An analysis of results from simulations with four different models in
NETCARE (Mahmood et al., 2016) indicates that the main source
of BC in the Arctic is long-range transport from mid-latitudes. The
long-range transport of BC to the Arctic is particularly efficient in
midwinter and then decreases in efficiency, reaching a minimum in March and
April. At the same time, dry deposition decreases, and wet deposition from
clouds in the low and mid troposphere becomes more important during the
transition from winter to spring. Overall, sources and sinks of BC in the
Arctic are well balanced, leading to nearly steady Arctic burdens during the
time period from December to May. Subsequently, during the transition from
spring to summer, precipitation increases and wet deposition becomes highly
efficient, which leads to substantial reductions in BC burdens in the Arctic
despite increased emissions from vegetation fires. At high altitudes in the
Arctic, the model results indicate that convective transport of pollution
from the lower to the upper troposphere at lower latitudes and subsequent
long-range transport to the Arctic represents an important source of BC.

Xu et al. (2017)
interpreted a series of airborne and ground-based BC measurements made using
multiple measurement techniques with the GEOS-Chem global chemical transport
model and its adjoint to attribute the sources of Arctic BC (Fig. 11). This
was the first comparison of BC measurements from a Single Particle Soot
Photometer (SP2) at Alert with a chemical transport model. The inclusion of
seasonally varying domestic heating and of gas-flaring emissions was crucial
to successfully simulating ground-based measurements of BC concentrations at
Alert and Barrow and airborne BC measurements across the Arctic. Sensitivity
simulations suggest that anthropogenic emissions in eastern and southern Asia
have the largest effect on the Arctic BC column burden in spring (56 %),
with the largest contribution in the middle troposphere. At the Arctic
surface, anthropogenic emissions from northern Asia (40 %–45 %) and
eastern and southern Asia (20 %–40 %) are the largest BC contributors in
winter and spring, followed by Europe (16 %–36 %). This dominant role of
Asian sources is consistent with some recent studies (e.g. Ma et al., 2013;
Wang et al., 2014; Ikeda et al., 2017) but differs from many earlier studies
(e.g. Stohl, 2006; Shindell et al., 2008; Gong et al., 2010; Huang et al.,
2010; Bourgeois and Bey, 2011; Sharma et al., 2013) due to decreased European
emissions and increased Asian emissions. The adjoint simulations enabled
identification of pronounced spatial heterogeneity in the contribution of
emissions to the Arctic BC column concentrations, with noteworthy
contributions from emissions in eastern China (15 %) and western Siberia
(6.5 %). The Tarim oilfield in western China stood out as a specific
influential source with an annual contribution of 2.6 %. Emissions from
as far away as the Indo-Gangetic Plain could have a substantial influence
(6.3 %) on Arctic BC as well.

Figure 11GEOS-Chem adjoint modelling results for BC sources to the Arctic.
Panels (a) and (b) show in colour the contributions of BC
from different anthropogenic emission and biomass burning regions to the
vertical profiles in the atmosphere, where the measurements are in black.
Modelled results are for the entire Arctic for the annual average. The data
are binned in pressure ranges in panels (c) and (d).
Numbers of measurements are along the y axis. Figure from Xu et
al. (2017).

4.5 Springtime aerosol: optical properties

Kodros et al. (2018) combined measurements of BC mixing state in the
springtime Canadian high Arctic with simulated size-resolved aerosol mass and
number concentrations to constrain model estimates of the direct radiative
effect (DRE). Airborne measurements using an SP2 (soot particle photometer) and Ultra-High
Sensitivity Aerosol Spectrometer on board the Polar 6 aircraft show
an average coating thickness of 45 to 40 nm for BC core diameters across the
range of 140 to 220 nm, respectively. For total particle diameters ranging
from 175 to 730 nm, BC-containing particle number fractions range from
16 % to 3 %. GEOS-Chem-TOMAS yields a pan-Arctic average springtime DRE for all aerosols ranging from
−1.65 W m−2 when assuming entirely externally mixed BC to
−1.34 W m−2 when assuming entirely internally mixed BC. Using the
observed mixing-state constraints from this field campaign significantly
reduces this estimated range in DRE by over a factor of 2 (−1.59 to
−1.45 W m−2). Measurements of mixing state thus provide important
constraints for model estimates of the DRE.

Some of the first vertically resolved and concurrent measurements of aerosol composition
and optical properties in the springtime high Arctic are presented in Leaitch et
al. (2019). As shown in Fig. 12a, observations from the Polar 6 during April
2015 indicate an increase in the fraction of refractory black carbon (rBC) in submicron
particles with altitude coincident with an increase in the overall carbonaceous fraction
of the submicron aerosol for flights conducted around Alert, Nunavut, and Eureka, Nunavut
(Schulz et al., 2019; Willis et al., 2019). For values of the light scattering
coefficient (σscat) less than 15 Mm−1, which represent 98 % of
the measured σscat during the Alert and Eureka flights, the single
scattering albedo (SSA) of the aerosol decreases from 0.96 near the surface to 0.93 at
500 hPa (Fig. 12a). The SSA decrease with altitude is consistent with the increasing
fraction of rBC in the particles and suggests a stabilizing influence of BC on the high
Arctic atmosphere. In an absolute sense, the σscat values primarily vary
with the sum of ammonium, organics, and sulfate as shown in Fig. 12b.

Figure 12(a) Vertical profiles of the ratio of organic material to sulfate
(Organic/sulfate) from Willis et al. (2019), the ratio of refractory black carbon (rBC)
to the volume concentration of the submicron aerosol estimated from the measured size
distribution (Schulz et al., 2019) and one estimate of the single scatter albedo (SSA; Leaitch et
al., 2019). (b) Profiles of the sum of the mass concentrations of ammonium
(NH4), organic material (Org.) and sulfate (SO4) with the light
scattering coefficient (σscat). All values are medians over
approximately 50 hPa pressure intervals. Results are for flights conducted out of Alert
and Eureka, and constrained to σscat<15 Mm−1, which represents
98 % of the observed σscat.

4.6 Monitoring the transitions between seasons by remote sensing

While in situ field campaigns provide detailed information over a short period of time,
remote sensing provides annual measurements and thus information about the transitions
from winter to spring and then into summer. In particular, ground-based lidar and star
photometry (carried out at the PEARL observatory in Eureka, Nunavut) and satellite-based
lidar (CALIOP/CALIPSO) during the latter half of two polar winters suggest the frequent
Arctic-wide presence of submicron particles in the boundary layer with aerosol optical
depths (AODs) significantly greater than the AOD predicted by GEOS-Chem, in which the AOD
largely results from sulfate particles (O'Neill et al., 2016). Ground-based sun
photometry (AEROCAN/AERONET) measurements acquired between 2009 and 2012 at five
western-Arctic stations (Hesaraki et al., 2017) revealed Arctic-wide springtime peaking
of both submicron and super-micron AODs that were roughly consistent with submicron and
super-micron AOD estimates from GEOS-Chem (predominantly associated with Arctic haze
sulfates and Asian mineral-dust aerosols, respectively). A summertime peak in submicron
particles, which was determined to be smoke induced at the four westernmost AEROCAN
Arctic stations, was not simulated by GEOS-Chem. Rather, GEOS-Chem indicated a continuous
spring-to-fall decrease in submicron AOD that was principally associated with a decrease
in sulfate contributions.

4.7 Aerosol deposition to snow

Deposition fluxes in the Arctic are very poorly characterized, in large part because of
the logistical challenges of collecting continuous data series. To address this, NETCARE
scientists collected temporally resolved data for the chemical composition of snow
(common metals, BC, soluble ions, and small organics) that fell throughout the cold
season at Alert (Macdonald et al., 2017). In particular, new snow was collected after
each appreciable period of precipitation, resulting in samples every 4 days, on average,
from September 2014 to May 2015.

Using measurements of the amount of snow that had fallen in a given area, the chemical
compositions were converted to fluxes for comparison with modelled values. In combination
with ambient air concentrations of the equivalent chemical species, the measured fluxes
were then expressed as an effective deposition velocity, which encompasses both wet and
dry deposition processes (Fig. 13; Macdonald et al., 2017). Interestingly, effective
deposition velocities are higher for the warmest months (September, October, May) than
for the cold months, arising from the switchover from liquid water to a combination of
dry deposition and ice cloud scavenging. The effective deposition velocities for BC were
the smallest of all species characterized, consistent with its low hygroscopicity and
poor ice-nucleating abilities.

To take advantage of the high temporal resolution of the samples, the data
were also used to assess potential sources contributing to chemical species
in snow using a combination of positive matrix factorization and FLEXPART
potential emission sensitivity analysis (Macdonald et al., 2018). The best
positive matrix factorization solution consisted of seven source factors
(sea salt, crustal metals, BC, carboxylic acids, nitrate, non-crustal
metals, and sulfate), reflecting a balance between natural and anthropogenic
sources. Notable findings include identification of anthropogenic sources
(but not biomass burning) as dominant for BC during this study period, and a
potential source of volcanic sulfur in the fall of 2014.

Figure 13The monthly average (circles) effective deposition velocity of
different chemical species to snow at Alert during 2014–2015. Median values
(bars) are also shown. The effective deposition velocity encompasses both wet
and dry deposition processes. In general, the warmer months have higher
deposition velocities than the colder months, likely due to enhanced wet
deposition in the former. Figure from Macdonald et al. (2017).

A simple parameterization of BC in snow was developed and tested in the
Canadian Atmospheric Global Climate Model (CanAM). According to the
parameterization, the temporal evolution of the concentration of BC near the
top of the snowpack is determined by changes in dry and wet deposition of BC,
the snowfall flux and scavenging by snow meltwater. Comparison of model
results with a multi-year climatology of BC concentrations in snow produces
good agreement for locations in the Canadian Arctic and sub-Arctic (Doherty
et al., 2010, 2014; X. Wang et al., 2013) as well as for other regions in the
Northern Hemisphere (Namazi et al., 2015). Simulated changes in BC loading in
snow in the second half of the 20th century had much smaller cryospheric
impacts on surface air temperatures than other aerosol and greenhouse gas
radiative forcings.

4.8 Ship emissions

Understanding the impacts of ship emissions on climate and air quality of the Arctic
environment is challenging but important, given the likelihood of future increases in
Arctic shipping (Corbett et al., 2010; Pizzolato et al., 2014; Winther et al., 2014). The
Arctic atmospheric boundary layer exhibits different dynamics from mid-latitudes, being
characterized by thermally stable conditions with reduced turbulent mixing (Aliabadi et
al., 2016a). Ships navigating northern latitudes operate under partial engine load and
ice-breaking conditions as opposed to full speed cruising. Uncertainties are compounded
by the lack of accurate predictions for increased ship traffic patterns in the Arctic as
the ice cover retreats, as well as the lack of a robust regulatory framework to control
emissions via emission control areas set by the International Maritime Organization
(Aliabadi et al., 2015).

The NETCARE campaign near Resolute Bay in July 2014 characterized typical ship emissions
and plume evolution by mapping the plume of the CCGS Amundsen with the
Polar 6 research aircraft (Aliabadi et al., 2016b). Three plumes were
intercepted, with the first corresponding to operation of the CCGS Amundsen in
open water under low-speed cruise conditions, while the second and third corresponded to
operation under ice-breaking conditions. The measured species included mixing ratios of
CO2, NOx, CO, SO2, particle number concentration,
BC, and CCN. Lower plume expansion rates were observed compared to mid-latitudes due to
reduced turbulent mixing, resulting in a poorly diluted plume that was confined within a
low boundary layer. Most, but not all, emission factors agreed with prior observations
for low engine loads at mid-latitudes. This implied different emission factors for each
species measured. In particular, ice-breaking increased the NOx emission
factor to values equivalent to those measured for high engine loads at mid-latitudes,
likely due to differences in engine combustion temperatures. The CO emission factor was
higher at low engine loads, whereas the BC emission factors were similar to those at
mid-latitudes; the effect of engine load on BC emission factors is still debated in the
literature. While various authors report increasing emission factors by decreasing
engine loading (Agrawal et al., 2008; Petzold et al., 2010, 2011; Khan et al., 2012),
other authors report decreasing emission factors by decreasing engine loading (Cappa et
al., 2014). Due to the use of low sulfur diesel fuel by the CCGS Amundsen, no
SO2 was detected.

5.1 Rationale and research questions

Currently, clouds are responsible for some of the greatest uncertainties in climate
change predictions. This is in large part because the properties of clouds and their
formation processes are poorly understood, especially in the case of ice and mixed-phase
clouds (Cantrell and Heymsfield, 2005; Hegg and Baker, 2009; Murray et al., 2012).
Particles that can initiate ice formation in the atmosphere at temperatures and relative
humidities lower than those required for homogeneous freezing of solution droplets are
referred to as ice nucleating particles (INPs; Vali et al., 2015). Only a very small
fraction of atmospheric particles (1 in 10−3 to 10−5) can act as INPs (Rogers
et al., 1998), but predictions of climate and precipitation can depend strongly on INP
concentrations (DeMott et al., 2010; Lohmann, 2002). As an example, an increase in the
concentrations of INPs can lead to more precipitation and shorter cloud lifetimes for
mixed-phase clouds, resulting in less solar reflectivity (DeMott et al., 2010; Lohmann,
2002). Despite the importance of INPs, the level of scientific understanding of their
concentrations and sources in the atmosphere remains low (Coluzza et al., 2017). To
improve predictions of precipitation and climate in the Arctic, the concentrations and
sources of INPs in this region need to be determined. This information can then be used
to test and constrain parameterizations in atmospheric models (Vergara-Temprado et al.,
2017).

5.2 INPs in the sea surface microlayer and bulk sea water

The sea surface microlayer is the interface between the atmosphere and the ocean and is a
source of particles to the atmosphere via wave-breaking and bubble-bursting. INPs have
previously been detected in bulk seawater (Schnell, 1977; Schnell and Vali, 1975, 1976);
however, concentrations and properties of INPs in the microlayer have not been confirmed
prior to the start of NETCARE. This lack of information led to large uncertainties in
quantifying the importance of the microlayer as a source of INPs to the atmosphere
(Burrows et al., 2013). In initial experiments, the concentration of INPs in the
microlayer collected off the west coast of Canada were measured (Wilson et al., 2015);
while in parallel, researchers from the University of Leeds measured the properties and
concentrations of INPs in the microlayer collected off the east coast of the United
States and Greenland (Wilson et al., 2015). We built on this work by measuring the
concentrations and properties of INPs in the microlayer collected in the Canadian Arctic
(Irish et al., 2017, 2018).

Microlayer samples were collected in the Canadian Arctic during the summers
of 2014 and 2016 from the CCGS Amundsen. INPs were ubiquitous in the microlayer with
freezing temperatures as warm as −5∘C. Concentrations of INPs
were higher on average in 2016 than in 2014 or off the east coast of the US
and Greenland (Wilson et
al., 2015). The INP concentrations were enhanced in the microlayer compared
to bulk seawater in several samples collected in 2016. Concentrations of
INPs were anti-correlated with salinity, possibly indicating that the INPs
were associated with melting sea ice. The INPs had diameters between 0.2 and
0.02 µm and were heat-labile, and therefore likely biological
material. Possible candidates for the INPs include exudates from sea-ice
microorganisms such as sea-ice diatoms and bacteria.

5.3 INPs in the high Arctic during spring–summer

The size of INPs collected from the atmosphere at Alert in late spring and early summer
2014 were also measured (Mason et al., 2016). The size of atmospheric INPs can help
distinguish which types of atmospheric particles are important as INPs. During this
campaign, the average concentrations of atmospheric INPs were 0.05, 0.22, and
0.99 L−1 at freezing temperatures of −15, −20, and −25∘C,
respectively. The median diameters of the INPs were 3.2, 2.2, and 0.83 µm at
freezing temperatures of −15, −20, and −25∘C, respectively, and the
average fractions of INPs ≥1µm were 95 %, 66 %, and 41 % at
freezing temperatures of −15, −20, and −25 ∘C, respectively. These results
suggest that the major sources of the INPs at this site during the collection period were
not submicron aerosol particles, such as ammonium sulfate, and BC particles. The sizes of
the INPs measured at Alert were consistent with those of INPs measured at five other
sites in North America, as well as one in Europe (Mason et al., 2016).

Figure 14Results from measurements in the Arctic marine boundary layer during summer 2014
(Irish et al., 2019). (a) Ratios of the surface area of mineral dust particles
to the surface area of sea salt particles measured by computer controlled scanning
electron microscopy with energy-dispersive X-ray spectroscopy (CCSEM-EDX). Ratios of
predicted INP concentrations from mineral dust, [INP(T)]MD, to the predicted
INP concentrations from sea spray aerosol, [INP(T)]SS, are calculated using
CCSEM-EDX measurements at temperatures of (b)−25∘C,
(c)−20∘C, and (d)−15∘C. Results show that
mineral dust is a more important contributor to the INP population in the Arctic than sea
spray aerosol at these times and locations.

During March 2016, INP measurements at Alert were made daily (Si et al.,
2018). In these high-frequency data, INP concentrations were strongly
correlated with tracers of mineral dust at a freezing temperature of −25∘C, suggesting that it was a major source of the INPs
measured at this temperature. These results are
consistent with the size of INPs measured at Alert during the spring and
summer of 2014. Particle dispersion modelling suggests that the mineral dust
may have been transported over long distances from the Gobi Desert.

5.4 INPs in the summertime marine boundary layer in the Canadian Arctic Archipelago

During the summer of 2014, we measured atmospheric concentrations of INPs in the Canadian
Arctic marine boundary layer on board the CCGS Amundsen (Irish et al., 2018).
Concentrations averaged 0.005, 0.044, and 0.154 L−1, at freezing temperatures of
−15, −20, and −25 ∘C, respectively. These values fell within the range of
atmospheric concentrations measured at locations outside the Arctic and in the marine
boundary layer (DeMott et al., 2016). Based on a combination of surface area measurements
of mineral dust and sea spray aerosol (Fig. 14) and particle dispersion modelling using
FLEXPART, mineral dust from local sources is a more important contributor than sea spray
aerosol to the atmospheric INP population for the times and locations studied. These
results do not rule out the sea surface microlayer as a source of INPs to the Arctic
marine boundary layer; rather, they show that the sea surface microlayer is likely a
smaller source of atmospheric INPs compared to local mineral dust for the locations and
times studied, at least at a freezing temperature of −25∘C.

Sulfuric acid coatings strongly affect INPs and thus their effect on clouds
and precipitation. This is particularly important during Arctic haze events.
Laboratory studies (Eastwood et al., 2009), in situ measurements (Jouan et
al., 2014), and large-scale observations from the CloudSat and CALIPSO
satellites (Grenier et al., 2009; Grenier and Blanchet, 2010) support the
hypothesis of a dehydration–greenhouse feedback (Blanchet and Girard, 1994)
linking acidified aerosols to the favoured formation of larger ice crystals
and light precipitation through a reduction of INP activity. In cold Arctic
conditions, thin ice clouds (TIC), like cirrus, are ubiquitous in the coldest
free troposphere (Grenier et al., 2009). Two types have been identified:
TIC-1, which has many small crystals (smaller than ∼30µm),
and TIC-2, which has fewer but larger ice crystals. While TIC-1 is largely
non-precipitating, TIC-2 leads to light precipitation, often in the form of
diamond dust, which is sometimes called clear sky precipitation because of
the very low optical depth of the clouds. Acidic INPs favour the formation of
TIC-2 in the middle and upper troposphere, which enhances water flux towards
the lower layers and leads to dehydration of the upper cold troposphere. In
turn, this process lowers the greenhouse effect of water vapour and favours
the direct IR cooling of the air mass in the lower atmosphere and at the
surface. Hence, variations in the INP composition can significantly affect
the radiative properties of the polar atmosphere and clouds, as well as the
atmospheric moisture concentration.

A far IR radiometer (FIRR) developed with Canadian Space Agency support and especially
designed to measure TIC properties and water vapour was flown on board the
Polar 6 aircraft during the NETCARE campaign of April 2015. The goal was to
simultaneously measure, for the first time, INPs, cloud microphysics, and spectrally
resolved radiation in the range of 8 to 50 µm over the Arctic. The experiment
successfully demonstrated closure between measurements and theoretical calculations for
clear-sky conditions (Libois et al., 2016a, b). It also showed the strong sensitivity of
FIRR observations to ice crystal size and cloud optical depth. However, the limited
number of ice clouds encountered and the complexity inherent in probing them with an
aircraft highlighted the need for further campaigns dedicated to simultaneous
investigation of ice cloud radiative and microphysical properties (Libois et al., 2016b).
The results obtained from the NETCARE campaign have paved the way for a future
satellite-based deployment over the poles, linking cloud microphysics, the atmospheric
water budget, and radiation balance.

The NETCARE research outlined above has provided novel insights into (1) the
biogenic sources of gases that can impact the size and composition of Arctic
aerosol; (2) new particle formation and growth into sizes that were
demonstrated to be activating to form cloud droplets, with growth occurring
largely via formation of Arctic marine secondary aerosol; (3) the sources and
properties of Arctic haze aerosol, in particular BC-containing particles; (4) Arctic INPs in the air, ocean water, and the sea surface microlayer; and
(5) deposition rates of pollutants to snow. Many of these advances arose as a
result of the highly interdisciplinary approach taken within NETCARE.
Nevertheless, despite these advances, many research questions remain, as
outlined in this section.

6.1 Marine and coastal biogenic aerosol precursors

Observations gathered during NETCARE provide a valuable benchmark upon which to base
predictions of the changes in the source strength of secondary MBA precursors in response
to alterations in Arctic climate. This is important for determining the amounts of both
aerosol sulfate and organics. The thinning and loss of seasonal sea ice, which is driven
by warming and polar amplification, is by far the most conspicuous of these alterations
(AMAP, 2013; Comiso, 2011; Serreze and Barry, 2011).

The marine production of DMS could be particularly sensitive to both modifications in
seasonal ice extent and the intermittent presence of melt ponds above the ice in spring
and summer (Gabric et al., 2017; Gourdal et al., 2018; Lizotte et al., 2019). Modelling
and observational studies suggest that the northward shrinking of the seasonal ice extent
and the ensuing increase in open waters available for gas exchange could lead to
heightened primary productivity (Arrigo and van Dijken, 2015; Gabric et al., 2017; Ito
and Kawamiya, 2010) and production of DMS (Levasseur, 2013). In turn, this would lead to
higher atmospheric MSA and secondary sulfate (Sharma et al., 2012), and background
particle concentrations (Dall'Osto et al., 2017). Simulations with CanAM indicate that
associated increases in concentrations of CCN could potentially offset part of the
warming due to enhanced cloud albedo (Mahmood et al., 2018). Specifically, the projected
loss of sea ice between 2000 and 2050 leads to a substantial increase in Arctic DMS
emissions in CanAM, leading the cloud radiative forcing associated with Arctic DMS to
increase from −0.13 to −0.27 W m−2 during this time period if marine
production of DMS is unchanged. Adding to these wide-ranging observations and modelling
outputs, NETCARE results suggest that as seasonal (i.e. first-year) sea ice becomes a
pan-Arctic feature in the future (AMAP, 2017), ice-related sources of DMS could increase
in response to the thinning of sea ice, as well as to the areal and temporal expansion of
melt ponds that act both as a source of DMS and as a catalyst of under-ice bloom
development. Conversely, observational and modelling studies also suggest that increased
stratification in ice-free waters of the Arctic could curb primary productivity due to
nutrient limitation (Steiner et al., 2015) and that wind-induced sea spray may be more
prevalent in open waters, acting as a condensation sink for material that could form
secondary aerosol (Browse et al., 2014)

The Arctic system may also be vulnerable to other changes, notably ocean acidification,
as well as amplified warming and freshwater inputs (AMAP, 2013; Yamamoto-Kawai et al.,
2009). An experimental assessment of the impact of ocean acidification on DMS-producing
planktonic communities of Baffin Bay during NETCARE (Hussherr et al., 2017) revealed that
DMS production may decrease by 25 % under end-of-the-century scenario reductions of
pH (ΔpHT=-0.48), confirming results observed in another Arctic study
in the Svalbard archipelago which showed a 35 % decrease in DMS production (Archer et
al., 2013). Other studies, however, have suggested that organisms thriving in Arctic
waters may already be resilient to moderate or acute natural fluctuations in pH,
exhibiting a high capacity to compensate for modifications in pH (Hoppe et al., 2018) and
no significant changes in DMS following acidification (Hopkins et al., 2018). Further
experimentation is needed to identify the underlying causes for these contrasting DMS
responses to ocean acidification in Arctic waters.

NETCARE illustrated for the first time the influence of ammonia emissions from seabird
colonies on not only atmospheric mixing ratios (Wentworth et al., 2016) but also new
particle formation, aerosol neutralization, and associated indirect effects on climate
(Croft et al., 2016a, 2018). How will these emissions evolve with climate change and
potential changes in wildlife populations (Weimerskirch et al., 2018), habitat, and
migratory patterns? NETCARE measurements from Alert suggest that Arctic soils may also be
an ammonia source (Murphy et al., 2019), perhaps reflecting the redistribution of ammonia
between different components of the Arctic land–ocean ecosystem. This highlights the
importance of including bidirectional fluxes in chemical transport models for species
that move readily between the land, atmosphere, and ocean.

Unlike lower-latitude marine boundary layers (Quinn and
Bates, 2011) particle nucleation and growth was frequently observed during
NETCARE campaigns in the boundary layer in Arctic marine and coastal
regions. The Arctic may behave differently for a number of reasons: (1) the
persistent temperature inversion lowers the rate of mixing of surface
emissions; (2) the summertime atmosphere has 24-hour sunshine to drive
photo-oxidation; (3) the condensation sink is particularly low; and (4) the
low temperatures facilitate molecular cluster formation. It is crucial to
assess the chemical components and particle formation mechanisms that
prevail in this distinctive environment. Particularly valuable will be
on-line mass spectrometric measurements of the chemical composition of the
smallest clusters and particles that form at the early stages of the
nucleation and growth process.

The growth of 5–50 nm particles into CCN size ranges was evident in
multiple NETCARE campaigns (Burkart et al., 2017a, b; Collins et al., 2017;
Willis et al., 2016, 2017). Surprisingly, much of the submicron aerosol mass
associated with growth was organic in composition (Burkart et al., 2017b;
Willis et al., 2016, 2017), providing additional support to the idea that
secondary organic aerosol of marine origin is important (Rinaldi et al.,
2010). Although we do not know the precise nature of the organic precursors,
the NETCARE studies described in Sect. 4.2 illustrated that the secondary
organic source is marine and potentially associated with oxidation or
photochemistry of the sea surface microlayer (see Sects. 3.3 and 6.3). It is
important to determine the balance between secondary aerosol formation versus
primary particle formation from sea spray. In one NETCARE case study of new
particle formation and growth over Lancaster Sound, there were indications of
secondary processes occurring alongside formation of sea spray salt
particles, suggesting that these processes may sometimes occur
simultaneously, complicating analyses (Collins et al., 2017; Köllner et
al., 2017; Willis et al., 2016). Key uncertainties in the radiative effects
of the Arctic MSOA simulation in GEOS-Chem-TOMAS include Arctic nucleation processes, the chemical
composition of Aitken particles, and the volatility of the SOA (Croft et al.,
2018). Further understanding of these processes would better constrain
climate feedbacks. We note that the composition and properties of Arctic MSOA
are not necessarily the same as that formed in marine environments in other
parts of the world.

6.3 The sea surface microlayer

Our understanding of how the sea surface microlayer impacts air–sea exchange of aerosols
and gases is still largely circumstantial and is based mainly on conceptual models (Garbe
et al., 2014; Lewis and Schwartz, 2013), laboratory experiments (Bigg and Leck, 2008;
Wilson et al., 2015), and observations of similarities between particulate matter in the
microlayer and the atmosphere (Leck and Bigg, 2005). Obtaining information on how these
concepts play out in the real world has proven extremely challenging. That being said,
one pronounced example of the potential importance of the sea surface microlayer comes
from work in NETCARE that demonstrated that OVOCs in a marine Arctic setting were likely
formed photochemically within the microlayer or by oxidation of gases arising from it
(Mungall et al., 2017). Although laboratory studies have previously demonstrated that
OVOCs can be chemically generated from microlayer surrogate materials (Rossignol et al.,
2016; Zhou et al., 2014), such studies do not address the chemical complexity of the
genuine environmental system.

Although additional experiments have previously quantified the impact of
microlayer surfactant enrichment on gas exchange (Brockmann
et al., 1982; Frew et al., 2004; Pereira et al., 2016), to date no one has
directly tied natural variations in the sea surface microlayer to the
exchange of aerosols or gases. The main difficulty lies in the different
temporal and spatial scales of atmospheric and microlayer measurements. The
composition of the microlayer is highly heterogeneous even on small
horizontal scales (Cunliffe et al., 2013), and
recovery of microlayer samples for chemical analysis is time consuming.
Thus, tying those measurements to observations of temporally variable
aerosols measured from ships or airplanes is innately subject to substantial
uncertainties.

In order to confidently identify the relationship between the composition of
the sea surface microlayer and atmospheric aerosol production, it will be
necessary to collect coherent data sets from single platforms, such as
autonomous surface craft (Ribas-Ribas et al., 2017). In
addition, intelligently designed time series stations could provide
sufficient data to identify clear relationships between the microlayer and
the atmosphere (Cunliffe et al.,
2013; Engel et al., 2017).

6.4 Removal of aerosol particles in the summertime

The 2014 NETCARE aircraft campaign illustrated that the low CCN numbers prevalent in the
summer boundary layer can lead to large cloud droplet diameters, in some cases
approaching 30 µm (Leaitch et al., 2016). The settling velocity of such
droplets is sufficiently fast that drizzling low-level clouds and fogs play an important
role in deposition to the surface (Browse et al., 2014). As yet, there is no Arctic
deposition network that is quantitatively assessing the importance and efficiency of such
processes. An important question that arises is the degree to which trends in wet
scavenging are driving the trends in aerosol loadings. It is well documented that both
aerosol sulfate and BC are currently lower in abundance than they were in previous
decades (AMAP, 2015). To what degree is this trend due to reductions in source emissions
as opposed to changes in cloud scavenging? In the summer in particular, the wider
expanses of open ocean associated with sea-ice melting may lead to higher water fluxes
from the ocean to the atmosphere, potentially affecting cloud liquid water content and
deposition rates. Model simulations suggest enhanced wet deposition of sulfate aerosol by
precipitation in reduced sea-ice conditions (Mahmood et al., 2018). It is not known
whether this enhanced deposition will affect sea-ice melt rates.

6.5 Cloud scavenging and long-range transport

As described in Sect. 2, there is a transition in scavenging regimes between
the efficient processes that occur with liquid clouds and the comparatively
inefficient processes associated with pure ice clouds. However, the community
struggles to accurately include such scavenging processes in models (Mahmood
et al., 2016). This is important for long-range transport of pollutants from
more industrialized southerly locations, and for the input of biomass burning
aerosol that is likely to become more prevalent with the warming climate
(Marelle et al., 2015; Shindell et al., 2008; H. Wang et al., 2013).

The degree of aerosol scavenging that occurs outside the Arctic relative to
that which occurs within it must be better established. For example,
transport associated with warm conveyor belt systems is one mechanism that
supplies pollutants to the Arctic (Ancellet et al., 2014; Roiger et al.,
2011), while cloud formation associated with synoptic uplift in mid-latitudes
cleans the air. This has been nicely demonstrated by a close inverse
relationship between the accumulated precipitation along back trajectories, a
measure of wet scavenging, and BC levels arriving in the Arctic (Matsui et
al., 2011). Deciphering the efficiency of such extra-Arctic processes is one
focus of the proposed IMPAACT project
(https://pacesproject.org/abstract/introducing-impaact-investigating-pollutant-transport-asia-arctic-and-north-america,
last access: 16 February 2019), which will involve multiple research aircraft
and surface-level vessels trying to better understand pollutant levels at
their sources and their modifications along these transport pathways. IMPAACT
is one effort of PACES which is a broader activity aimed at reducing
uncertainties associated with pollution in the Arctic
(https://pacesproject.org/about, last access: 16 February 2019). A
second example is wet scavenging that occurs as a result of air lifting over
Arctic terrain, such as Ellesmere Island and Greenland. In ongoing NETCARE
analysis, there is evidence that new particles formed in very clean free
tropospheric air masses that had been lifted over Greenland and passed to its
north. It is likely that cloud scavenging occurred over Greenland.

A related question is the degree to which oxidation processes modify the overall aerosol
composition as a function of residence time in the Arctic. For the measurements described
in Sect. 4.4, the increase in the sulfate-to-organic ratio with decreasing altitude in
springtime aerosol may in part arise from formation of sulfate as the air mass ages.
Validation of this mechanism awaits better SO2 measurements.

6.6 INPs in the cold seasons and atmospheric impacts

While aerosol particle removal is exceedingly efficient under summer
conditions, the ice nucleation processes in the colder months are much more
selective and less well understood, as described in Sect. 5. We sought to
understand which aerosol types contain the best ice-nucleating particles.
Initial indications from NETCARE measurements are that dust is an important
contributor to the INP population (Fig. 14), but that does not rule out the
role of primary sea spray particles acting as INPs. A second important
question was to what degree coatings of secondary materials, such as
sulfates or organics, modify the ice nucleation properties of primary INPs,
such as mineral dust. A major challenge is the development of better
parameterizations of INPs for use in atmospheric modelling. To that end,
work in NETCARE improved ice nucleation parameterizations in the Global
Multiscale Environmental Model (GEM-LAM) to determine the effect of
pollution on clouds in the Arctic (Keita and Girard, 2016).
To simulate pristine clouds, a parameterization of ice nucleation by mineral
dust was included, whereas to simulate ice clouds influenced by pollution, a
parameterization of ice nucleation by mineral dust coated with sulfuric acid
was used. A parameterization was developed as well to test against the 2014
and 2016 CCGS Amundsen INP measurement data. Nevertheless, many details about the
ice cloud formation process are still missing from these parameterizations.

Although NETCARE measurements of aerosol deposition fluxes to the snow at
Alert were made across a full cold season (see Sect. 4.7), the degree to
which this flux occurred via dry or wet deposition was not precisely
determined. In particular, it remains to be determined how important ice
cloud scavenging and settling is as a particle removal process. A long-term,
high time resolution aerosol deposition network that separates wet and dry
deposition across the Arctic would be highly beneficial in this regard.

6.7 Aerosol particle mixing state

Mixing state refers to the uniformity of the distribution of the aerosol chemical
components across an array of particles; i.e. are all the particles of the same chemical
composition or is their chemical distribution heterogeneous? NETCARE measurements have
highlighted how this information is crucial to our understanding of aerosol sources and
impacts. In particular, the springtime measurements of BC aerosol described in Sects. 4.4
and 4.5 showed that within the Arctic haze sampled in spring 2015, only 3 %–16 %
of the particles contained BC inclusions and that BC-containing particles had coatings
40–45 nm in thickness on average (Kodros et al., 2018). The direct radiative forcing
that is modelled using these results as constraints is distinctly different from that
where it was assumed that the chemical mixing state of the aerosol was uniform (see
Sect. 4.5). Likewise, in the summertime measurements from 2014, the single-particle mass
spectrometry measurements at low altitudes over open water illustrate that primary and
secondary marine aerosol components were externally mixed, thus indicating different
formation processes (Fig. 9; Köllner et al., 2017; Willis et al., 2016). More
measurements of this type, down to as small a particle size as possible, are crucial for
further determining the balance between primary and secondary marine aerosols, to
establishing the degrees of coating that exist on mineral dust aerosol that contain INPs,
and to assessing the efficiency of cloud scavenging that occurs across different particle
types. For example, does the relatively hydrophobic character of BC inhibit the rate at
which it is wet cloud scavenged, and if so, how much hygroscopic coating material must be
present to make the particles CCN active?

6.8 Measurements across the seasons and throughout the atmosphere

The Arctic springtime has been much more extensively studied than other seasons. This is
understandable given the importance of the Arctic haze phenomenon. However, the fall and
winter seasons are poorly characterized using intensive campaign approaches, largely
because of the operational difficulties in working under cold, dark conditions. Although
remote sensing can be used to study transitions between seasons (see Sect. 4.6), it is
still important to better understand how transport patterns of pollutants and their
deposition rates change seasonally. As well, we know very little about the polar night
and the associated formation of ice clouds. The radiative effects of these clouds and
their ability to dehydrate the atmosphere on a large scale through extensive light
precipitation are important to assess. An exciting movement in this direction is the
development of a far infrared radiometer (FIRR) that was flown on the Polar 6
aircraft for the first time within NETCARE (Libois et al., 2016a, b; see Sect. 5.5). By
providing improved ice cloud characterization and measurements of atmospheric water
vapour, this instrument can be used to improve understanding of the cooling of the
atmosphere via infrared emissions in cold polar regions.

The vertical profiles measured as part of NETCARE in both the springtime and summertime
provide essential information for comparison to model outputs and provide a necessary
complement to the much more extensive sets of measurements from ground-based field
campaigns and stations. Additional aircraft campaigns that provide such vertically
resolved features are a top priority for future studies. Nevertheless, the continuous
measurements at Arctic ground stations remain our most valuable data set to assess
long-term trends. There is a significant need to enhance the instrumental capabilities at
these stations, for example with key continuous measurements of SO2,
NH3, VOCs, and aerosol composition across all size ranges to further our
understanding of many of the issues described above.

The NETCARE atmospheric measurements are publicly available
through the Government of Canada open data portal
(https://open.canada.ca, last access: 16 February 2019, Government of
Canada, 2019) and the oceanic measurements are available via the Polar Data
Catalogue (https://www.polardata.ca/, last access: 16 February 2019,
Canadian Cryospheric Information Network, 2019).

JPDA coordinated and wrote substantial portions of the paper. All other
co-authors contributed text and/or reviewed the paper. All co-authors either
wrote a first-author paper as part of the NETCARE project or else
contributed in a substantive manner to the research conducted in the project
or presented in the paper. EG contributed to the INP and ice cloud research
of NETCARE but he died before submission. We regard his approval for
inclusion of his name on this paper as implicit.

This article is part of the special issue “NETCARE (Network on
Aerosols and Climate: Addressing Key Uncertainties in Remote Canadian
Environments) (ACP/AMT/BG inter-journal SI)”. It is not associated with a
conference.

NETCARE was funded by the Natural Sciences and Engineering Research Council (NSERC) of
Canada under its Climate Change and Atmospheric Research program, with additional
financial and in-kind support from Environment and Climate Change Canada, Fisheries and
Oceans Canada, the Alfred Wegener Institute, and the Major Research Project Management
Fund at the University of Toronto. Colorado State University researchers were supported
by the US Department of Energy's Atmospheric System Research, an Office of Science,
Office of Biological and Environmental Research program, under grant no. DE-SC0011780,
the US National Science Foundation, Atmospheric Chemistry program, under grant
no. AGS-1559607, and by the US National Oceanic and Atmospheric Administration, an Office
of Science, Office of Atmospheric Chemistry, Carbon Cycle, and Climate Program, under the
cooperative agreement award no. NA17OAR430001. All authors would like to strongly thank:
(i) the editors for the NETCARE special issue in Atmospheric Chemistry and Physics, Biogeosciences, and Atmospheric Measurement Techniques for
their time and commitment, (ii) members of the NETCARE Scientific Steering Committee, and
(iii) other NETCARE collaborators.

This paper is dedicated to Eric Girard, a NETCARE scientist who died 10 July 2018.
Eric Girard contributed greatly to the field of Arctic cloud and aerosol microphysics
during his research career.

Aliabadi, A. A., Staebler, R. M., and Sharma, S.: Air quality monitoring in
communities of the Canadian Arctic during the high shipping season with a
focus on local and marine pollution, Atmos. Chem. Phys., 15, 2651–2673,
https://doi.org/10.5194/acp-15-2651-2015, 2015.

Jiao, C. and Flanner, M. G.: Changing black carbon transport to the Arctic
from present day to the end of the 21st century, J. Geophys. Res.-Atmos.,
121, 4734–4750, https://doi.org/10.1002/2015JD023964, 2016.

The Arctic is experiencing considerable environmental change with climate warming, illustrated by the dramatic decrease in sea-ice extent. It is important to understand both the natural and perturbed Arctic systems to gain a better understanding of how they will change in the future. This paper summarizes new insights into the relationships between Arctic aerosol particles and climate, as learned over the past five or so years by a large Canadian research consortium, NETCARE.